0.05%.

"FDA approved for the treatment of acne, it's also FDA approved for facial wrinkles and is used to treat hyperpigmentation. Multiple clinical studies have shown that topical tretinoin improves the visible signs of skin aging including fine lines, uneven pigmentation, and rough skin texture. One particularly important study followed patients for 2 years who were using 0.05% tretinoin once a day. That study showed that long-term use of topical 0.05% tretinoin daily was not only safe, but was effective for improving the visible signs of moderate to severe sun damage. The research doesn't suggest that a higher percentage strength is any better than a lower percentage strength. Higher percentages of topical tretinoin can work faster, but they also can be significantly more irritating and that irritation can actually cause hyperpigmentation for some people. For other people it makes it intolerable and they bail on the topical. They get too much dryness, too much peeling, too much flaking."

"Tretinoin is the big guns here, it's prescription only. It works on fine lines and wrinkles, hyperpigmentation, keeps our pores clear, and thus helps prevent acne breakouts. But it can also treat acne. You can find prescription tretinoin traditionally at 0.025, 0.05, and 0.1%. For me, I can't go any stronger than the 0.05. I stick with this and I do just fine. Tretinoin is prescription only because it requires no conversion steps to become retinoic acid, whereas retinaldehyde is only one conversion step away. All you need is a pea-sized amount. If you apply more than that, you're going to run into trouble. There's no way that applying more retinoid is going to make it work faster or make it work better at cleaning out your pores or for fine lines and wrinkles. It's just going to increase your risk for irritant contact."

"This is kind of the gold standard, this is what a lot of the research has been done on, is the 0.05% tretinoin. When you look at studies, it's shocking how irritating this can be for some people and how non-irritating this can be, and the results are very similar in the study. In terms of my best overall prescription retinoids, we're looking at tretinoin of various degrees. I do use all of these at various times. I'll put a dollop of this into like an SA cream, in a scoop in my hand, rub that together, and that becomes my body lotion."